A potential clinical aspect of the lubricity of the SCL is how it may change upon prolonged wear. To simulate this, the SCLs were cycled in and out of a tempered TLF solution for 18 hours. The cycling is believed to mimic the disturbance of the stratified tear-film structure that occurs during a blink,
51 and its effects on surface deposits. Interestingly, the bulk lens material did not seem to influence, nor be predictive of changes occurring in CoF upon ageing. Instead, the presence of a wetting agent appears crucial in retaining a low CoF after prolonged exposure to TLF. For example, among all the lenses that contained PVP, none showed an increase in CoF after ageing, regardless of whether the lens was made from a SiHy or hydrogel material (
Fig. 8). Lens DD_5, which contains PVA, poly (ethylene glycol) (PEG) and hydroxypropylmethylcellulose (HPMC) as wetting agents, showed an increase in CoF after ageing. This could be related to a depletion of these wetting agents over time, which is inherent in the design of this particular lens.
52 Among the lenses that have a surface coating, RU_4 was the only lens that did not show any increase in CoF upon ageing. This can be compared to the large increase seen for RU_1 and may be said to be an unexpected result as the two lenses are both silicone hydrogels with a plasma-treated surface. However, the plasma treatment of RU_4 has been reported to result in a hydrophilic layer of approximately 25 nm that prevents protein adsorption in the bulk.
20 In contrast, RU_1 has an incomplete plasma coating consisting of hydrophobic islands that could lead to increased deposits with a higher degree of denatured proteins and, as a result, a higher CoF.
29 In fact, RU_4 displayed a slight decrease in CoF upon ageing—a difference that was removed as the CoF measurement proceeded (0, 50, and 100 cycles). This may indicate the presence of a thin film of loosely adsorbed species with inherently lower CoF compared to the pristine lens surface, as also observed by Ngai et al.
9 after lysozyme adsorption. Lens DD_1 has a hydrogel coating at the interface.
49 The CoF of this lens has been reported previously to be approximately 0.02 in PBS against a silica sphere,
53 in agreement with our results before ageing. Dursch et al.
49 characterized DD_1 with regards to solute-partitioning, and demonstrated the hydrogel characteristics of the lens surface, as well as showing that the gel layer has a negative charge at neutral pH. The increase in CoF after ageing could be a consequence of electrostatic adsorption of proteins, or an increased crosslink density of the hydrogel due to association of divalent cations present in the lubricant – similar to what was observed by Dunér et al.
54 for poly (acrylic acid) polymer brushes in the presence of calcium. Certain lenses had an increase in CoF after prolonged exposure to TLF. This may be of significance when attempting to correlate CoF with SCL comfort after wear.